Integrated circuit fabrication processes include steps in which some materials should be selectively removed through either wet or dry etching process. Regardless of the method used, the selectivity between the materials that are removed and those that are not removed should be high.
Silicon nitride is a dielectric material that is very commonly applied in the manufacture of semiconductor devices. The wet etching process is commonly used for removing silicon nitride in the presence of oxides. In general, silicon nitride on a semiconductor substrate is selectively etched by soaking the substrate in a hot phosphoric acid bath for a period of time. Etching silicon nitride by phosphoric acid may be described by the following equation:Si3N4+4H3PO4+10H2O→4NH4H2PO4+9Si3O2(OH)8 
In this equation, the silicon atom is oxidized and thus forms hydrous silica (Si3O2(OH)8) in solution, which inhibits the etching of silicon oxide so as to increase selectivity of silicon nitride to silicon oxide. In order to increase silicon concentration of the hot phosphoric acid bath to obtain enough high selectivity, a number of dummy wafers, each having a silicon nitride layer, may be soaked in the bath for a period of time before processing the semiconductor substrate, which results in additional preparation time and manufacturing cost. Given the above, it remains desirable to provide an etching process without soaking the dummy wafers in the bath before processing the semiconductor substrate.